Various stationary and mobile tanks are used in the production, storage and distribution of volatile organic compounds such as fuels, solvents and chemical feedstocks. When transferring a volatile fuel such as gasoline from a fixed roof storage tank to a fixed roof receiving tank; two events simultaneously occur. Vapors in the receiving tank ullage (space above the liquid) are displaced by the incoming liquid, and a negative pressure in the storage tank is developed in response to the dropping liquid level. The negative pressure in the storage tank is offset by either the ingestion of atmospheric air; or in the case of facilities equipped with Stage II vapor recovery systems, a hydrocarbon/air mixture. If the hydrocarbon concentration in the storage tank ullage is reduced below the naturally occurring equilibrium concentration dictated by the volatility and temperature of the fuel, a driving force for evaporation of valuable liquid gasoline is established. As the storage tank liquid evaporates to re-establish the equilibrium hydrocarbon concentration in the ullage space, the volume expansion of liquid to vapor measures approximately 520:1, and the resulting large volume of vapor is exhaled until equilibrium is achieved. These emissions are comprised of VOC's (Volatile Organic Compounds) which are ozone precursors and hazardous air pollutants (HAPS) such as benzene. These gasoline vapor emissions represent an economic loss to the retailer, an environmental hazard to both air and groundwater and a negative impact on human health since benzene is a known human carcinogen.
Accordingly, vapor losses from fixed-roof gasoline storage tanks includes displacement losses caused by inflow of liquid, breathing losses caused by temperature and atmospheric pressure variations, and emptying losses caused by evaporation of liquid after the transfer of product occurring during the interval between the next product delivery
Capture of displacement losses in the United States petroleum industry has been addressed by Stage I, Stage II and ORVR vapor recovery systems. The Stage I systems return vapors displaced from the large capacity storage tanks to the ullage space of the high volume tanker truck. Stage II systems return vapors displaced from vehicle fuel tanks to the storage tanks, and ORVR (On-board refueling Vapor Recovery) systems capture vapors displaced from vehicle fuel tanks within a canister, located within the vehicle, containing selectively adsorbent material.
A major concern among regulators and petroleum marketers alike is ensuring that hundreds of thousands installed storage tank and vapor recovery systems are performing effectively over an on-going, continuous interval. The Stage I systems and Stage II systems prevent emissions of VOC's and HAPS to the environment. A properly engineered, manufactured and installed storage tank system prevents the leakage of liquid phase product into groundwater. Recently Mr. Gary Lynn from the New Hampshire DES has conducted research which shows a clear link between groundwater contamination and elevated storage tank pressures. The high tank pressures result in “below grade vapor emissions” which eventually condense and find their way into groundwater. In addition the State of Maryland has recently enacted emergency regulations to mitigate this same problem. The problems in Maryland were amplified by two high profile release incidents where large volumes of liquid phase gasoline were leaked from fueling stations into groundwater which comprised residential drinking wells for nearby residents.
To measure storage tank leak integrity and Stage I and Stage II system efficacy, petroleum marketers have made substantial investments in storage tank and product line leak detection systems, Automatic Tank Gauges (ATG's), Statistical Inventory Reconciliation (SIR, SIRA and CSLD) algorithms, and so-called ISD (In-station diagnostic systems). All of the automated systems rely on tank gauges (or manual sticking of tanks) to provide raw data inputs into their various algorithms and sophisticated computer software calculations. Ostensibly, these hardware devices and software algorithms appear effective in meeting the above needs. However, upon closer examination, the existing products and services have significant disadvantages.
The key governing equation for storage tank systems is as follows:INPUT−OUTPUT=ACCUMULATION   (1)If the owner or operator of a gasoline refueling station is confident that liquid leaks are not present, the other means of apparent or measured loss of mass are through evaporation loss, meter miscalibration, invoice errors, theft or volumetric changes due to temperature variation. Variations of these techniques have been evaluated by various independent third party testing organizations and have subsequently been approved by NWGLDE (National Work Group on Leak Detection Evaluations) for tank monitoring protocols designed to detect liquid leaks and thereby avoid major environmental spills and their associated costly remediation.
However, for the material balance to generate accurate results, temperature compensation is necessary to avoid significant calculation errors caused by volume growth or contraction of liquid gasoline. It is known in the art that typical gasoline blends experience a volume change of approximately 0.70% upon undergoing a temperature change of 10 F. There are two significant impacts of temperature and volume variation in the context of tank gauging and inventory reconciliation.
The first error caused by a change in specific volume (density) of the fluid being measured causes the float used in magnetostrictive probes to register an inaccurate reading. The magnetostrictive probe uses a physical height reference of a float supported along the length of a rod to provide the level and associated volume of the fluid in the tank. For a 10,000 gallon tank with an eight foot diameter and length of twenty-seven feet; at half height of 4 feet of product in the tank, there are 135 gallons of fuel per inch of product height. Since the weight of the float is supported by the buoyancy force equal to the weight of the fuel displaced by the float, a change in density of the fuel must yield a slightly different height of the float along the length of the probe. A small error of only 0.10 inches of height will yield a corresponding error of 13.5 gallons of liquid volume in just a single tank. For a three tank system of 30,000 gallons, the total measurement error at a single point in time can easily be 40.5 gallons. In Equation (1) above, the ACCUMULATION term will be erroneous.
The second error caused by the temperature and density variation of fuel results in an inaccurate and non-repeatable measurement of dispensed fuel. Dispenser meters in the United States are not temperature compensated such as those in other countries such as Canada. The pulse meter within the dispenser therefore measures fuel volumes at the prevailing temperature of the fuel reaching the pulse meter. Depending on tank and piping burial depth, piping run length, solar impacts, fuel pumping profile, forecourt construction material, and submersible turbine pump type, the temperature of the fuel arriving at the pulse meter within the dispenser is not exactly equal to the temperature of the fuel being withdrawn from the storage tank. Unless these temperatures are exactly the same, the raw data inputs will be comprised of poor quality data. No matter how sophisticated the algorithms are within the various vendor supplied packages for inventory reconciliation and leak detection, the results are doomed to failure based on poor quality raw data inputs. In Equation (1) above, the OUTPUT term will be in error.
Based on the large number of L.U.S.T. (Leaking Underground Storage Tank) sites throughout the United States, the overall deficiency of past storage tank containment algorithms can be somberly confirmed. It is perhaps not well known, but diesel fuel is the fluid used in a majority of the third party tank gauge and automated system tests reviewed by the NWGLDE. Perhaps if gasoline was used as the test fluid, many of the systems and/or algorithms receiving commercial approval at very low detection limits would be found inappropriate. Perhaps the various inventory control systems provide a business management tool, but reliance on existing algorithms for detailed leak detection and regulatory compliance does not seem to be a wise choice.
In terms of providing insight into the efficacy of Stage I and Stage II vapor recovery systems, so-called ISD systems are being proposed. These ISD systems make use of vapor flow meters installed in at least each gasoline dispenser to measure the ratio of returned vapor to dispensed liquid fuel. These systems are complicated and costly to retrofit to existing fueling stations. In addition, these systems rely on mathematical algorithms to make inferences about vapor collection rates when more than one nozzle is being used to refuel a vehicle from the same dispenser. In addition, these systems require integration to additional electronic gear typically comprising an automated tank gauge system.
Typically, petroleum marketers do not believe they are losing fuel in the form of vapors to the atmosphere. The direct measurement of the vapor loss can be accomplished by the use of a flow meter installed on the vent lines of storage tanks. However, the installation of such flow meters at a large number of refueling sites is not practical, and another simpler means is required to readily calculate the vapor generation rates within the storage tanks over a wide range of conditions. Also, the mere installation of the flow meter device itself represents additional leak sources within the vapor piping at the refueling facility.